1. Field of the Invention
This invention relates to a display device and associated drive method, and more particularly comprises a display panel arranged with a plurality of display pixels which have current control type optical elements related to a display device which displays desired information and the method for driving the display device.
2. Description of the Related Art
In recent years, the increase of flat panel type display devices as monitors and displays of personal computers and video equipment has been amazing. Particularly, Liquid Crystal Displays (hereinafter denoted as “LCD”) have advanced rapidly as these devices are thin-shaped, space-saving, low-powered and the like as compared to conventional display devices. In addition, relatively small LCD's remarkably have also recently spread and are widely applied as display devices in such as cellular/mobile phones, digital cameras, Personal Digital Assistants (PDA's) and the like.
Furthermore, as the display device (display) of the next generation following such an LCD, Research and Development (R&D) of the self-luminescence type display device (hereinafter denoted as a “self-luminescence type display”) comprised of a display panel with optical elements arranged in a matrix form consisting of self-luminescence type light emitting devices, such as organic electroluminescent devices (hereinafter denoted as “organic EL devices”), inorganic electroluminescent devices (hereinafter denoted as “inorganic EL devices”) or Light Emitting Diodes (LEDs) and the like is being actively pursued. In comparison with former LCD's, such a self-luminescence type display has a more rapid display response speed and does not have a limited viewing angle. Additionally, as high luminosity increases contrast, higher resolution display image quality using low-power and the like are realistic. Because backlight is not needed like an LCD, this very predominant feature will lead to more thin-shaped and lightweight models and full-scale utilization of such self-luminescence type displays are expected in the near future.
In the configuration which applied an active-matrix drive method in the above-mentioned self-luminescence display, optical elements are added that are composed of the above-mentioned light emitting devices. Each of the display pixels constitutes the display panel. In addition, the drive method comprises a the driver circuit (hereinafter denoted as a “pixel driver circuit” for convenience) is composed of a plurality of switching element for performing drive control of these optical elements. A configuration which drives the light emitting devices of each display pixel is known, and the drive method of the circuit configuration of a pixel driver circuit or by means of light emitting devices has been variously proposed.
FIG. 10 shows an example of a circuit configuration of prior art of a display pixel in the self-luminescence type display comprised with an organic EL device as the light emitting device.
In the display pixel of prior art, for example, as shown in FIG. 10, the pixel driver circuit DCP with optical elements comprises a Thin-Film Transistor (TFT) Tr31 and a Thin-Film Transistor Tr32. The Thin-Film Transistor Tr31 gate terminal is connected to the selection lines SL, along with the source terminal and the drain terminal each other connected to the data lines DL and contact point N31 (hereinafter denoted as “contact” for convenience of explanation) each near the intersecting point of a plurality of selection lines SL (scanning lines) and data lines DL (signal lines) arranged in a matrix form in the display panel. The Thin-Film Transistor Tr32 gate terminal is connected to contact N31 and the source terminal each other connected to the ground potential Vgnd and an optical element. The anode terminal is connected to the drain terminal of the Thin-Film Transistor Tr32 of a pixel driver circuit DCP and the cathode terminal is connected to the constant voltage Vss lower than the ground potential Vgnd. This light emitting device is constituted of an organic EL device OEL which performs luminescent operation according to the applied current.
Also, a parasitic capacitance Cp is provided between the gate-source of the Thin-Film Transistor Tr32. Furthermore, the Thin-Film Transistor Tr31 is constituted by an n-channel type MOS transistor (NMOS). The Thin-Film Transistor Tr32 is constituted by a p-channel type MOS transistor (PMOS).
Additionally, in the pixel driver circuit DCP which has such a configuration, the Thin-Film Transistors Tr31 and Tr32 are switched “ON” at predetermined timing and drive control of the organic EL device OEL performs an “OFF” control.
Thus, in the pixel driver circuit DCP, initially, when a high-level selection signal Vsel is applied to the selection lines SL, the display pixels are set to a selection state by the scanning driver and the Thin-Film Transistor Tr31 performs an “ON” operation. The signal voltage Vpix is applied to the data lines DL by the data driver according to the display signal and applied to the gate terminal of the Thin-Film Transistor Tr32 via the Thin-Film Transistor Tr31. Accordingly, the Thin-Film Transistor Tr32 performs an “ON” operation as a result of the switch-on state according to the above-mentioned signal voltage Vpix. The drive current according to the signal voltage Vpix flows in the direction of the constant voltage Vss via the Thin-Film Transistor Tr32 and the organic EL devices OEL from the ground potential Vgnd. This drive current is supplied to the organic EL devices OEL and light is emitted by the luminosity gradation according to the display signal.
Secondly, when a low-level selection signal Vsel is applied to the selection lines SL and the display pixels are set to a non-selection state, the Thin-Film Transistor Tr31 performs an “OFF” operation. The data lines DL and the pixel driver circuit DCP are electrically blocked out. Thereby, the voltage applied to the gate terminal of the Thin-Film Transistor Tr32 is stored by the parasitic capacitance Cp and the Thin-Film Transistor Tr32 maintains an “ON” state. The operation in which the drive current flows to the organic EL devices OEL via the Thin-Film Transistor Tr32 from the ground potential Vgnd is maintained and the luminescent operation is continued. This luminescent operation is controlled, for example, so that one frame periods are continuously performed until the signal voltage Vpix is written in each display pixel according to the display signal.
Since such a drive method controls the current value of the drive current flow to the light emitting devices by regulating the voltage applied to each display pixel and performs a luminescent operation by predetermined luminosity gradation, it is called a voltage drive method or the voltage application method.
However, in the display device comprised with the display pixels of the pixel driver circuit which was mentioned above, it has a problem as illustrated below.
Specifically, in the pixel driver circuit as shown in FIG. 10, when the device characteristics, such as channel resistance and the like, in the two Thin-Film Transistors Tr31 and Tr32 and the device characteristics, such as electric resistance and the like, in the organic EL devices OEL change attribute properties with the passage of time according to the surrounding temperature and operating time, the drive current supplied to the light emitting devices will fluctuate and the luminescent luminosity of the light emitting devices will vary. Thereby, the luminosity gradation characteristics of the light emitting devices in contrast with the display signal change and result in the problem of not being able to acquire stable display image quality over a long period of time.
Additionally, because the variation in operating characteristics, such as the current between source-drain of the Thin-Film Transistors Tr31 and Tr32 which constitute the pixel driver circuit, becomes greater when each of the display pixels that constitute the display panel is miniaturized to attain a higher-resolution display image quality, proper gradation control becomes complicated to resolve. Thus, the problem of variation occurring in the display properties of each display pixel causes deterioration of the image quality.
Furthermore, in the pixel driver circuit as shown in FIG. 10, since the ground potential Vgnd serves as the current supply source connected to the source terminal of the Thin-Film Transistor Tr32 which supplies drive current to the light emitting devices in the above circuit configuration and the constant voltage Vss of low electric potential is connected to the cathode side of the light emitting device rather than the current supply source, in order to operate these Thin-Film Transistors satisfactorily, it is necessary to apply a PMOS transistor. However, when a Thin-Film Transistor is provided using amorphous silicon by means of manufacturing technology already established, there is difficulty in actualizing a PMOS transistor with sufficient operating characteristics and functional capability. Thus, when a pixel driver circuit has an integrated configuration with PMOS transistors, the manufacturing technology of polysilicon or single crystal silicon must to be used. Nevertheless, in the manufacturing technology using polysilicon or single crystal silicon as compared with the manufacturing technology using amorphous silicon, the assembly process is more complicated and the production cost is expensive. Thus, there is the drawback of causing a sharp increase in the product cost of a display device comprised with the pixel driver circuit.